1. Field of the Invention
The present invention relates to a method for operating a reference speed in an anti-lock braking control of an automobile having two or more wheels, and an anti-lock braking control technique on the basis of the reference speed.
2. Description of the Related Art
As a brake control unit for a vehicle such as a two-wheeled vehicle, an anti-lock braking system (ABS) is publicly known which prevents the elongation of a braking distance of the vehicle or the impossibility of the control of the traveling direction of the vehicle with its steering wheel owing to the slipping of the vehicle caused by the locking of a wheel at the time of quick braking or at the time of braking on a slippery road surface.
The anti-lock braking system performs a brake control in order not to lock wheels by detecting the rotational speeds of the wheels at the time of the braking caused by a certain braking operation, and performs the brake control by performing the pressure intensifying control or the pressure reducing control of the brake operation hydraulic pressures to the wheel cylinders of the vehicle on the basis of the detected rotational speeds. To put it concretely, when a wheel is going to be locked, the anti-lock braking system repeats the control of decompressing the brake operation hydraulic pressure to each of the wheel cylinders and the control of pressurizing the brake operation hydraulic pressure according to the wheel rotational speeds to prevent the locking of the wheel. The control of decompressing the brake operation hydraulic pressure is performed by opening the pressure reducing electromagnetic valve on the communicating path of each of the wheel cylinders and each of reservoirs to feed a part of the brake operating liquid, which is fed from each of the master cylinders to each of the wheel cylinders, to each of the reservoirs to decompress the brake operation hydraulic pressure to each of the wheel cylinders. The control of pressurizing the brake operation hydraulic pressure is performed by closing the pressure reducing electromagnetic valve with the brake operating liquid fed from the master cylinder to the wheel cylinder. Moreover, the brake operating liquid fed to each of the reservoirs is compulsively fed to each of the master cylinders by a pump.
When the pressure intensifying control and the pressure reducing control of the brake operation hydraulic pressures to each of the wheel cylinders is performed on the basis of the rotational speed of each of the wheels in the anti-lock braking control, control variables such as acceleration and deceleration of the wheels, a slip ratio, a pseudo car body speed and reference body deceleration are operated by an operation block in an electric control unit (ECU) in the anti-lock braking system. Then, a control block in the ECU performs a brake control in order not to lock the wheels by performing the pressure intensifying control and the pressure reducing control of the brake operation hydraulic pressures to each of the wheel cylinders on the basis of the operated control variables.
The slip ratio, one of the control variables, can be operated from the wheel speed of each of the wheels and a car body speed. When the car body speed and the wheel speed are equal, the slip ratio is 0%. However, the slip ratio becomes larger as the difference between them becomes larger at the time of braking, and it becomes 100% when the wheel is locked before the stop of the car body. However, because the actual speed of the car body cannot be measured, when the slip ratio is operated, it is general that a reference body speed is operated and the slip ratio is operated by supposing the operated reference speed as the car body speed.
For example, in a motorcycle, the reference speed is generated from the wheel speed of an undriven wheel, i.e. the front wheel of the motorcycle, at the time when the anti-lock braking control of the motorcycle is not performed. The reason why the reference speed is generated from the wheel speed of the undriven wheel is that, first, in the state in which the driving wheel of the motorcycle is coupled with an engine, the moment of the inertia of the undriven wheel is sufficiently smaller than that of the driving wheel and is more sensitive to changes of braking torque, and consequently, at an initial step at the time of braking, the reference speed generated from the undriven wheel (front wheel) is closer to the car body speed at that time. Moreover, because the rear wheel being the driving wheel has the possibility of a rapid slip at the time of acceleration, the reference speed generated on the basis of the wheel speed of the front wheel is closer to the actual car body speed.
On the contrary, at the time of the anti-lock braking control, the reference speed having a larger value among the two values of the reference speeds generated on the basis of each of the wheel speeds of the front wheel and the rear wheel (driving wheel), namely the reference speed generated on the faster wheel speed among the wheel speeds of the front wheel and the rear wheel, is selected. The selection is generally called as the so-called select-high (or high-select), and is a method for operating a reference speed based on a publicly known technical basis. The method is regarded as the most effective method for approximating the progress of the reference speed to the actual car body speed to grasp the slip ratio as accurately as possible.
Now, particularly in a motorcycle, a phenomenon easily occurs which the rear wheel of the motorcycle floats at the time of braking. Among motorcycles, a vehicle having a larger ratio of the height of the center of gravity of the vehicle to a distance between the wheel shafts of the front wheel and the rear wheel has the inclination that the rear wheel of the vehicle easily floats. Then, when only the front wheel of such a motorcycle is braked while traveling on a road surface having a large coefficient of friction (the so-called high xcexc road surface), the rear wheel of the motorcycle easily floats in the air. From the point of view of achieving the safety of a traveling vehicle, also, it is desired to prevent such lifting of the rear wheel by detecting the lifting of the rear wheel to perform, for example, a suitable brake control of the front wheel.
However, for example, if braking is performed only to the front wheel and the rear wheel floats during the anti-lock braking control of the front wheel, the lifting of the rear wheel removes the ground resistance to the rear wheel to decrease the deceleration of the rear wheel rapidly. Consequently, if the anti-lock braking control is executed on the basis of the reference speed operated by following the wheel speed of the rear wheel selected in accordance with the above-mentioned select-high, the reference speed becomes higher than the actual car body speed at the time of the lifting of the rear wheel. Thereby, the situation is judged to be one in which a wheel is slipping relatively. Then, although the wheels are not slipping, a pressure reducing control of a wheel cylinder is performed. Consequently, the problem in which the braking feeling of a brake is spoiled is produced.
Accordingly, it is possible to prevent the situation in which a reference speed becomes higher than an actual car body speed by taking a basis on the reference speed operated to follow the wheel speed of the front wheel selected by a method, the so-called select-low, to select the value of a reference speed being a smaller one among the values of the reference speeds of the front wheel and the rear wheel at the time of the lifting of the rear wheel. However, in this case, the selected reference speed becomes lower than the actual car body speed in turn. Consequently, there is the possibility of occurring the problem in which extreme changes of vehicle stability occur in the time of traveling on a low xcexc road surface particularly.
The present invention was made in consideration of such situations. It is an object of the present invention to realize an anti-lock braking control having little possibility of decreasing the vehicle stability on a low xcexc road surface while decreasing the degree of the deterioration of the braking feeling of a brake which deterioration is produced by the conventional uniform setting of a reference speed to a wheel speed in accordance with the selection of the select-high at the time of the lifting of a rear wheel of the vehicle.
For achieving the object, a first aspect of the present invention, is a method for operating a reference speed in an anti-lock braking control of an automobile having two or more wheels, which method comprises the steps of: supposing a wheel speed of a wheel rotating at a slowest rotational speed to be VRL; supposing a wheel speed of a wheel rotating at a fastest rotational speed to be VRH; supposing a fixed default to be xcex1; and operating the reference speed to follow a wheel speed VRM calculated in accordance with a following formula (1).
VRM=xcex1VRL+(1xe2x88x92xcex1)VRH (0 less than xcex1 less than 1)xe2x80x83xe2x80x83(1) 
As described above, if the anti-lock braking control is executed on a reference speed operated in accordance with the select-high, namely a reference speed operated to follow the wheel speed VRH of a wheel rotating at the highest rotational speed, the problem of spoiling the braking feeling of a brake at the time of the lifting of the rear wheel is produced. On the other hand, if the anti-lock braking control is executed on a reference speed operated in accordance with the select-low, namely a reference speed operated to follow a wheel speed VRL of a wheel rotating at the lowest rotational speed, the reference speed becomes lower than the actual car body speed at the time of traveling on a low xcexc road surface particularly, in turn. Then, there is the possibility of generating the problem such that extreme changes occur in the aspect of the vehicle stability.
In the above-mentioned formula (1), the default xcex1 is fixed to a value within a range being larger than 0 and smaller than 1. That is, the wheel speed VRM becomes a wheel speed between the wheel speed VRH of the wheel rotating at the fastest speed and the wheel speed VRL of the wheel rotating at the slowest speed. Consequently, for example, if the value of the default xcex1 is fixed to be 0.5, the wheel speed VRM becomes a wheel speed right in the middle between the wheel speed VRH of the wheel rotating at the fastest speed and the wheel speed VRL of the wheel rotating at the slowest speed. Then, if the value of the default xcex1 is made to be larger than 0.5, the wheel speed VRM becomes a wheel speed closer to the wheel speed VRH of the wheel rotating at the fastest speed. And if the value of the default xcex1 is made to be smaller than 0.5, the wheel speed VRM becomes a wheel speed closer to the wheel speed VRL of the wheel rotating at the slowest speed.
In such a way, the value of the default xcex1 is fixed to be an arbitrary value within the range of 0 less than xcex1 less than 1, and the reference speed is made to follow the wheel speed VRM calculated in accordance with the formula (1). Thereby, it becomes possible to approximate the reference speed at the time of the lifting of the rear wheel to the actual car body speed more closely. Incidentally, because the easiness of the lifting of a rear wheel differs according to a ratio of the height of the center of gravity of a vehicle and a distance between the wheel shafts of the front wheel and the rear wheel, different values of the default xcex1 are fixed according to the easiness of the lifting of vehicles, and accordingly the value of the default xcex1 is obtained by experiments or the like.
Thereby, according to the method for operating a reference speed of the first aspect of the present invention, because the reference speed at the time of the lifting of a rear wheel can be approximated to an actual car body speed more closely by making the reference speed follow the wheel speed VRM, the operation and the advantage of the realization of an anti-lock braking control having the following feature can be obtained. That is, the feature is that the anti-locking braking control has little possibility of decreasing the vehicle stability at the time of traveling on a low xcexc road surface while decreasing the degree of the deterioration of the braking feeling of a brake which deterioration is produced by the conventional uniform setting of a reference speed to a wheel speed by the selection in accordance with the select-high at the time of the lifting of the rear wheel of the vehicle.
A second aspect of the present invention is an anti-lock braking control method of an automobile having two or more wheels, which method comprises the step of operating a reference speed in accordance with the method for operating the reference speed of the first aspect.
According to the anti-locking braking control method of the second aspect of the present invention, the above-mentioned operation and the advantage of the first aspect of the present invention can be obtained in the anti-locking braking control method.
A third aspect of the present invention is an anti-lock braking control method according to the second aspect, which method further comprises the steps of: operating the reference speed to follow the wheel speed VRH during an anti-lock braking control; noticing reference body deceleration calculated on a basis of the reference speed to judge that lifting of a rear wheel from a supporting surface has occurred if the reference body deceleration is equal to a predetermined value or more during the anti-lock braking control of a front wheel; and operating the reference speed to follow the wheel speed VRM at a time of the lifting of the rear wheel.
According to results of driving tests and the like and the characteristics of the configurations of the bodies of motorcycles, it is known that a rear wheel is easy to float in the case where car body deceleration exists in a region where the car body deceleration exceeds a predetermined value. Accordingly, by noticing reference body deceleration calculated from a reference speed made to follow the select-high during an anti-lock braking control of the front wheel, it can be presumed that the lifting of the rear wheel easily occurs when the reference body deceleration is equal to the predetermined value or more.
Thereby, according to the anti-lock braking control method of the third aspect of the present invention, in addition to the operation and advantage of the second aspect of the present invention, the operation and the advantage can be obtained in which it is possible to deal with the lifting of the rear wheel appropriately and make the reference speed, which is made to follow the wheel speed VRH ordinarily, follow the wheel speed VRM precisely when the possibility of the lifting of the rear wheel is high, because it is considered that the lifting of the rear wheel has occurred and then it is judged that the lifting of the rear wheel has occurred in the state in which the lifting of the rear wheel can be pressured to occur easily.
A fourth aspect of the present invention is an anti-lock braking control method according to the second or the third aspect, wherein a value of the default xcex1 is variable according to a state of the anti-lock braking control.
As described above, because the value of the default xcex1 is variable according to the state of the anti-lock braking control, it becomes possible to set the wheel speed VRM, which the reference speed is made to follow, to be the most appropriate wheel speed according to car body speeds and the state of each wheel under various brake control conditions.
Thereby, according to the anti-lock braking control method of the fourth aspect of the present invention, in addition to the operation and the advantage of the second or the third aspect, the operation and the advantage can be obtained in which it is possible to realize the anti-lock braking control having little possibility of the decrease of the vehicle stability at the time of traveling on the low xcexc road surface while reducing the degree of the deterioration of the braking feeling of a brake, which deterioration is caused by making the reference speed follow the wheel speed selected in accordance with the select-high, under various brake control conditions, because it becomes possible to set the wheel speed VRM, which the reference speed is made to follow, to be the most suitable wheel speed.
A fifth aspect of the present invention is an anti-lock braking control method according to any one of the second to the fourth aspects, wherein the reference speed is operated to follow a wheel speed of an undriven wheel while the anti-lock braking control is not executed.
Thereby, according to the anti-lock braking control method of the fifth aspect of the present invention, in addition to the operation and the advantage of any one the second to the fourth aspects, because the reference speed generated from the wheel speed of the undriven wheel is closer to the actual car body speed when the anti-lock braking control is not performed as described above, the operation and the advantage can be obtained in which the reference speed can be set to the closest value to the car body speed when the anti-lock braking control is not performed.
A sixth aspect of the present invention is an anti-lock braking system of an automobile having two or more wheels, which automobile is equipped with wheel speed sensors generating rotational speed information of the wheels and an ECU including means for calculating wheel speeds on a basis of the rotational speed information of the wheels input from the wheel speed sensors to operate a reference speed on a basis of the wheel speeds for operating slip ratios and acceleration and deceleration of the wheels on a basis of the reference speed, wherein the ECU includes means for operating the reference speed to follow a wheel speed VRM calculated in accordance with the above-mentioned formula (1) by supposing a wheel speed of a wheel rotating at a slowest rotational speed to be VRL, and supposing a wheel speed of a wheel rotating at a fastest rotational speed to be VRH, and further supposing a fixed default to be xcex1.
According to the anti-lock braking system of the sixth aspect of the present invention, the operation and the advantage similar to those of the above-mentioned second aspect of the present invention can be obtained.
A seventh aspect of the present invention is an anti-lock braking system according to the sixth aspect, wherein the ECU operates the reference speed to follow the wheel speed VRH during an anti-lock braking control, and the ECU notices reference body deceleration calculated on a basis of the reference speed to judge that lifting of a rear wheel from a supporting surface has occurred if the reference body deceleration is equal to a predetermined value or more during the anti-lock braking control of a front wheel, and further the ECU operates the reference speed to follow the wheel speed VRM at a time of the lifting of the rear wheel.
According to the anti-lock braking system of the seventh aspect of the present invention, the operation and the advantage similar to those of the above-mentioned third aspect of the present invention can be obtained.
An eighth aspect of the present invention is an anti-lock braking system according to the sixth or the seventh aspect, wherein a value of the default xcex1 is variable according to a state of the anti-lock braking control.
According to the anti-lock braking system of the eighth aspect of the present invention, the operation and the advantage similar to those of the above-mentioned fourth aspect of the present invention can be obtained.
A ninth aspect of the present invention is an anti-lock braking system according to any one of the sixth to the eighth aspects, wherein the ECU operates the reference speed to follow a wheel speed of an undriven wheel while the anti-lock braking control is not executed.
According to the anti-lock braking system of the ninth aspect of the present invention, the operation and the advantage similar to those of the above-mentioned fifth aspect of the present invention can be obtained.
A tenth aspect of the present invention is a reference speed operation program for making a computer installed in an ECU execute an operation of a reference speed in an anti-lock braking system of an automobile having two or more wheels, which automobile is equipped with wheel speed sensors generating rotational speed information of the wheels and the ECU including means for calculating wheel speeds on a basis of the rotational speed information of the wheels input from the wheel speed sensors to operate the reference speed on a basis of the wheel speeds for operating slip ratios and acceleration and deceleration of the wheels on a basis of the reference speed, which program comprises the steps of: supposing a wheel speed of a wheel rotating at a slowest rotational speed to be VRL; supposing a wheel speed of a wheel rotating at a fastest rotational speed to be VRH; supposing a fixed default to be xcex1; and operating the reference speed to follow a wheel speed VRM calculated in accordance with the above-mentioned formula (1).
According to the reference speed operation program of the tenth aspect of the present invention, the operation and the advantage similar to those of the above-mentioned first aspect of the present invention can be obtained. Moreover, the reference speed operation program can bring about the operation and the advantage similar to those of the above-mentioned first aspect to the anti-lock braking system equipped with the ECU capable of executing the reference speed operation program.
An eleventh aspect of the present invention is an anti-lock braking control program for making a computer installed in an ECU execute an anti-lock braking control in an anti-lock braking system of an automobile having two or more wheels, which automobile is equipped with wheel speed sensors generating rotational speed information of the wheels and the ECU including means for calculating wheel speeds on a basis of the rotational speed information of the wheels input from the wheel speed sensors to operate the reference speed on a basis of the wheel speeds for operating slip ratios and acceleration and deceleration of the wheels on a basis of the reference speed, which program comprises the steps of: supposing a wheel speed of a wheel rotating at a slowest rotational speed to be VRL; supposing a wheel speed of a wheel rotating at a fastest rotational speed to be VRH; supposing a fixed default to be xcex1; and operating the reference speed to follow a wheel speed VRM calculated in accordance with the above-mentioned formula (1).
According to the anti-lock braking control program of the eleventh aspect of the present invention, the operation and the advantage similar to those of the above-mentioned second aspect of the present invention can be obtained. Moreover, the anti-lock braking control program can bring about the operation and the advantage similar to those of the above-mentioned second aspect to the anti-lock braking system equipped with the ECU capable of executing the anti-lock braking control program.
A twelfth aspect of the present invention is an anti-lock braking control program according to the eleventh aspect, which program further comprises the steps of: operating the reference speed to follow the wheel speed VRH during an anti-lock braking control; calculating reference body deceleration on a basis of the reference speed; judging that lifting of a rear wheel from a supporting surface has occurred if the reference body deceleration is equal to a predetermined value or more during the anti-lock braking control of a front wheel; and operating the reference speed to follow the wheel speed VRM at a time of the lifting of the rear wheel.
According to the anti-lock braking control program of the twelfth aspect of the present invention, the operation and the advantage similar to those of the above-mentioned third aspect of the present invention can be obtained. Moreover, the anti-lock braking control program can bring about the operation and the advantage similar to those of the third aspect to the anti-lock braking system equipped with the ECU capable of executing the anti-lock braking control program.
A thirteenth aspect of the present invention is an anti-lock braking control program according to the eleventh or the twelfth aspect, which program further comprises the step of changing a value of the default xcex1 according to a state of the anti-lock braking control.
According to the anti-lock braking control program of the thirteenth aspect of the present invention, the operation and the advantage similar to those of the above-mentioned fourth aspect of the present invention can be obtained. Moreover, the anti-lock braking control program can brought about the operation and the advantage similar to those of the fourth aspect to the anti-lock braking system equipped with the ECU capable of executing the anti-lock braking control program.
A fourteenth aspect of the present invention is an anti-lock braking control program according to any one of the eleventh to the thirteenth aspects, which program further comprises the step of operating the reference speed to follow a wheel speed of an undriven wheel while the anti-lock braking control is not executed.
According to the anti-lock braking control program of the fourteenth aspect of the present invention, the operation and the advantage similar to those of the above-mentioned fifth aspect of the present invention can be obtained. Moreover, the anti-lock braking control program can bring about the operation and the advantage similar to those of the fifth aspect to the anti-lock braking system equipped with the ECU capable of executing the anti-lock braking control program.
According to the present invention, an anti-lock braking control can be realized which has little possibility of decreasing the vehicle stability at the time of traveling on a low xcexc road surface while decreasing the degree of the deterioration of braking feeling of a brake which deterioration is produced by the conventional uniform setting of a reference speed to a wheel speed in accordance with the selection of the select-high at the time of the lifting of a rear wheel of the vehicle.